Hot runner nozzles often have an uneven distribution of heat along the length of the melt channel when operating in an injection molding apparatus. The nozzles are generally housed in an opening in a mold plate between a manifold and a mold cavity. The mold plate is typically cold, particularly near the manifold and the mold cavity. As a result, a nozzle head portion and the area around a nozzle tip tend to lose more heat through direct contact with the mold than the nozzle mid-section, which does not contact any part of the mold plate. The uneven distribution of heat along the nozzle causes the temperature of the melt flowing through the nozzle to vary as it travels toward the mold cavity. Any variation in melt temperature can adversely affect the quality of the molded products and is therefore undesirable.
A further disadvantage of an uneven temperature distribution along the length of the nozzle is that the nozzle is subjected to high stress due to the continuous cycling between higher and lower temperatures. This can result in a shorter nozzle life.
With the increased use of plastic materials that are more sensitive to fluctuations in temperature, the melt must be maintained within more accurate and controllable temperature ranges. If the temperature rises too high, to compensate for the heat loss through the contact with the mold, degradation of the melt will result; and if the temperature drops too low, the melt will clog in the system and produce an unacceptable product. Both extremes can necessitate the injection molding apparatus being shut down for a clean-out, which can be a very costly procedure due to the loss of production time.
Thermal sleeves have been used in attempts to reduce the uneven temperature distribution of injection molding hot runner nozzles. A sleeve of material that has a thermal conductivity greater than the material of the nozzle body is slid over the nozzle body and heater. Heater elements often have a circular or elliptical cross-section. Where such a heater is wound around the nozzle body and partially inserted in a groove, the sleeve only contacts the heater element and the contact with the heater element is only tangential. The tangential contact of the sleeve and heater element results in inefficient conduction between the components. Where the heater element is fully inserted in a groove, the sleeve usually only contacts the nozzle body. However, where the sleeve contacts the nozzle body and the heater element, the contact with the heater element is still only tangential and inefficient. The tangential contact results in the creation of an insulating dead air space between the sleeve and nozzle body.
Another hot runner nozzle includes a heating element inserted in a spiral groove on a nozzle body and the heating element is vacuum cast in copper. The process for making the nozzle requires the heating element to be inserted in the groove and spot welded. Then, a stainless steel sleeve is placed over the nozzle body and the heating element. A heating element is recessed within the spiral groove so that a space is created within the spiral groove between the outer surface of the heating element and the inner surface of the stainless steel sleeve. A filling reservoir that holds a copper slug is welded to an end of the nozzle so that the reservoir communicates with an opening in the spiral groove. The assembly is then placed in a vacuum furnace, and upon heating, the copper slug melts and the copper flows into the spiral groove filling the space between the heating element and sleeve. The stainless steel sleeve may then be machined off. Where the sleeve is removed, the copper does little to disperse heat along the length of the nozzle body because the copper only contacts the nozzle body within the spiral groove. If the sleeve is left in place, although the copper may improve conduction to the sleeve, the sleeve itself has a low thermal conductivity. In either case, the step of copper casting results in a painstaking and costly process.
Another hot runner nozzle design that has attempted to provide improved heat distribution includes a heating element that is cast in a conductive material, such as brass or beryllium-copper, and slid over a nozzle body. The cast heater has also been combined with a layer of conductive material that is coated on the outer surface of the nozzle. The heating element has to be cast during a separate operation. In addition, once the heater is cast there is less flexibility in changing the length of the casting. In addition, where a coating is utilized, the coating must be applied to the outer surface of the nozzle in a separate process. After those processes are completed, the nozzle would still require assembly. As a result, manufacturing such a hot runner nozzle is costly and time consuming.
Furthermore, thermally conductive coatings have also been utilized on the outer surface of hot runner nozzles and heaters to improve the thermal conductivity. However, coating processes must be properly monitored to assure that the coatings remain consistent and adequate.
There is therefore a need to provide a hot runner nozzle having a generally uniform temperature distribution along the length thereof.